Generally, refrigeration is used for temperature requirements from 80 – 85oF to as near absolute zero as the process demands. The petrochemical and chemical range does not go much below -200oF. Refrigeration process consists of five component equipment, they are:
1. Compressor.
2. Evaporator.
3. Condenser.
4. Expansion Valve.
5. and Refrigerant.
Process Description
A general equipment arrangement of Refrigeration cycle shows below:
1. Expansion. Liquid refrigerant is flashed across a control valve or other device to a lower temperature and pressure become two phases. The enthalpy of the resulting liquid and vapor must be the same as the enthalpy of the initial liquid or isenthalpic (vertical line on the pressure-enthalpy diagram on figure).
2. Evaporation. The two phase fluid formed in the expansion step is sent to a heat exchanger (typically called a chiller or evaporator) with low pressure and temperature then the cold refrigerant and the warm process gas exchange heat. The process fluid temperature decreases but the refrigerant’s temperature stays constant as it evaporates (heat absorbed by the refrigerant equal to its latent heat of vaporization).
3. Compression. The saturated vapor from the previous step is sent to a compressor, which compresses the vapor to a higher temperature and pressure. Frictional losses in the evaporator and piping cause the suction vapor to be slightly superheated.
4. Condensation. The high pressure superheated vapor from the compressor is then cooled and condensed at a relatively constant pressure. The cooling medium in the condenser is usually water, air, and another refrigerant depending on the condensation pressure required. Then it is ready to be sent back to the expansion step to start the cycle over again.
REFRIGERATING GROUPS
The most widely used design, all proposed by KASRAVAND are:
1. Mechanical Refrigeration.
2. Steam Jet Refrigeration.
3. Absorption Refrigeration.
All of the methods are using similar process to produce refrigeration effect: Evaporation, Condensation, and Expansion. The difference between them is in the way compression is done to the refrigerant.
The mechanical refrigeration used a Compressor, the Absorption (and desorption) process used the thermal energy forces, while the Steam Jet process used pressure difference from Ejector.
One of the most well-known refrigeration methods for the mechanical refrigeration is Vapor- Compressor Systems or sometimes also called Vapor-Compression Cycles. Isothermal processes are realized through isobaric evaporation and condensation in the tubes.
Work that produced in turbine is small. Therefore, the turbine equipment could be replaced by an expansion valve for efficiency. Nonetheless, wet compression is substituted to a compression vapor for the reasons of proper compressor function.
Absorption refrigeration is a form that is economically attractive when there is a source of thermal energy at a temperature between 100 to 200°C. An example of inexpensive thermal energy sources are, solar energy, waste heat from cogeneration or process steam plants, geothermal energy, and natural gas when available at a low price.
The absorption refrigeration involves the absorption process of a refrigerant by a transport medium called absorbent to form a liquid solution. The liquid solution the pumped to the higher pressure. Due to the average specific volume of the liquid solution is much less than of the refrigerant vapor, significantly less work is needed. Accordingly, absorption refrigeration system has the advantage of relatively small work input compared to vapor compression system.
Absorption refrigeration systems differ from compressor refrigerating systems at least in two major aspects:
1. The refrigerant drawn out of the evaporator is compressed not by a mechanical process, but by a thermal heat-exchange which consists of an absorber, a generator, and a pump.
2. Operation of thermal compressors requires a solvent (for a second working fluid) as well as refrigerant.
Two most generous industrial absorption-type refrigeration systems are:
1. Aqua ammonia system.
2. Lithium bromide-water system.
Both of the system respectively used ammonia and water as the refrigerant. Ammonia – water is used for evaporating temperature on the range -60 until 10°C, while water-lithium bromide is used for evaporating temperatures more than 0°C. No other any pair components had been suggested that widely used except the two of them.
The steam-jet refrigeration system could be defined as a thermally drive compression system in which vapor is compressed in a jet apparatus.
